This invention relates to co-axial magnetrons. A typical co-axial magnetron as at present known is represented in highly schematic manner in FIGS. 1 and 2 of the accompanying drawings of which FIG. 1 is a section in plan and FIG. 2 is a section in elevation. Referring to FIGS. 1 and 2, the magnetron consists of two concentric cylinders, an outer cylinder 1 and an inner cylinder 2, coaxial with the longitudinal axis of the magnetron. Between cylinders 1 and 2 is the main cavity 3 of the magnetron. The main cavity 3 is short-circuited at either end by shorting end plates 4 and 5 extending towards the outer and inner cylinders 1 and 2.
The inner cylinder 2 comprises an anode member of the magnetron and has a plurality of inwardly extending radial vanes or partitions 6 which define between them a multiplicity of subsidiary cavities 7, alternate ones of which are coupled to the main cavity 3 by means of axially extending slots 8 extending through the wall of the inner cylinder 2.
One problem suffered by co-axial magnetrons as described above is unwanted oscillation in what are termed "slot modes". The axially extending coupling slots 8 have but slight electrical excitation in the intended mode of oscillation but have large currents along their lengths in the unwanted "slot modes" of oscillation. These large currents flow up one edge of the slot and down the other.
The usual method of attenuating such unwanted "slot modes" is to introduce a thin cylinder of resistive material co-axial with and inside the inner cylinder 2 arranged to overlap the ends of the coupling slots 8. Usually such attenuating cylinders of resistive material are provided at both ends of the cylinder 2 as shown in FIG. 2 where the cylinders of resistive material are represented by the numeral 9.
If as is often the case currents induced in the resistive attenuating material of the cylinder 9 by the currents in the slots result in heating then the cylinders tend to expand into the wall of the inner cylinder 2 (which is usually of copper) which then conducts the heat away for dissipation.
It has been found that utilising this form of attenuation has two disadvantages particularly when applied to magnetrons of high average power or long wavelength (two factors which usually occur together). Firstly the relatively large size of each cylinder of resistive attenuating material (which is usually brittle) can result in fractures occurring under conditions of rapid or uneven heating and secondly the cylinders of resistive attenuating material can absorb power from currents in the wanted mode of oscillation which currents flow up the inside of the inner cylinder 2 on the metal between the attenuating slots 8. One object of the present invention is to reduce or avoid such difficulties.